PARENT SESSION
Oral Session 6: Herbivory I: Dynamics, Communities, and Photosynthesis.
Presiding: C Ivey
Monday, August 2, 8:00 AM to 11:30 AM, Meeting Room B 114.
Bottom-up and top-down forces mediated by plant ontogeny: Consequences for herbivores and plants.
Boege, Karina*,1, Marquis, Robert1, 1 University of Missouri- St. Louis, St. Louis, Missouri, USA
ABSTRACT- Traits influencing plant quality as food and/or shelter for herbivores may change during plant development. As a consequence, herbivore density and the amount of herbivore damage may be expected to change as plants develop. However, the role of plant ontogeny in tritrophic interactions has been the focus of little previous investigation. The goal of this study was to assess the influence of plant ontogeny for the relative importance of bottom-up and top-down forces, and to quantify their cascading effects on herbivory, growth and reproduction of the tropical tree Casearia nitida. Bottom-up forces were manipulated by fertilizing saplings and reproductive trees, while top-down forces were manipulated by excluding insectivorous birds. Total phenolics were higher in saplings than in reproductive trees, while fertilization increased nitrogen concentration in plants of both stages. Insect density was affected by ontogenetic variation in plant quality (total phenolics or unmeasured traits) and not by fertilization. Bird exclusion increased herbivore density on both stages, but the relative importance of top-down impacts was greater in saplings than in reproductive trees, while bottom-up forces had a greater impact for reproductive plants. Cascading effects of top-down forces increased herbivory on both ontogenetic stages, but had little or a slightly positive effect on plant growth. However, bird-excluded reproductive trees tended to produce fewer fruits than control trees, suggesting that insectivorous birds may influence the reproduction of this tropical tree. Together, our results suggest that plant ontogeny is one factor that should be incorporated into tritrophic interaction theory, as ontogeny can modify the impact of both bottom-up and top-down impacts on herbivores.
Key words: bottom-up vs. top-down, herbivory, ontogeny, cascading effects